Orifice meters, using piezometric methods to determine the differential head across a fixed orifice a conduit though which a fluid flows are well known in the prior art. A fixed orifice flow meter, however, has a very narrow range of flow rates over which accuracy of measurement can be maintained. If the orifice is small enough to provide good resolution of head differentials at low flow rates, the orifice will restrict much higher flow rates and produce erroneous high flow rate measurement. On the other hand, if the orifice is large enough to accommodate large flow rates there will be poor resolution of pressure differentials at low flow rates and consequent inaccurate measurements.
It is therefore the primary object of the present invention to provide a variable area orifice associated with the generation of differential pressure signal for use as a fluid flow meter.
A second objective of the invention is provide flow measuring apparatus having substantial pressure differentials across the measuring orifice for a wide range of rates of flow in order to achieve accurate measurements at both high and low flow rates.